1. Field of the Invention
This invention relates to an apparatus for driving for expansion or contraction a rod antenna element of an expansion antenna to be suitably used on an automobile or the like and, more particularly, it relates to an improvement made on the rotary drum of an expansion antenna for reeling or unreeling a rope for expansion or contraction of the antenna.
2. Description of the Related Art
The rotary drum of an apparatus for driving a rod antenna element of an expansion antenna for expansion or contraction of the above described type is normally located coaxially with and adjacent to a rope feeding wheel (or a pinion when a rope provided with a rack is used) so that a base portion of the rope for expanding or contracting the antenna device may be wound onto or unwound from a rope storing area inside the outer peripheral wall of the drum. Such a conventional drum is normally freely rotatable relative to its shaft so that it may rotate to follow the movement of the rope base portion (which is freely movable) independent of the rotary motion of the rope feeding wheel which is normally driven by an electric motor.
The structural design of a conventional rotary drum as described above is based on the fact that, if the rotary drum is also driven by a motor so that it may rotate in synchronism with the rotation of the rope feeding wheel, there arises a difference between the peripheral speed of the rope storing area of the rotary drum and the rope feeding rate of the rotary drum to give rise to undesirable friction there. The rotary drum is inevitably made to freely rotate in order to get rid of this undesirable friction.
A conventional rotary drum of the above identified type is accompanied by certain problems as described below. As the inner surface of the rotary drum loses its original smoothness with a prolonged use, the load of winding a rope to be borne by the drum is increased. The inner surface of such a rotary drum is normally coated with a layer of grease and, therefore, well lubricated. So, the rope will move smoothly sideways in a direction parallel to the axis of the drum to ensure an even and smooth winding or unwinding operation. If the rotary drum is used for a long period of time, however, the grease is moved away from the inner surface by silt and/or sand carried in on the peripheral surface of the rope which is frequently wound and unwound so that the inner surface of the rotary drum gradually loses its original smoothness and becomes coarse. Under this condition, the operation of unwinding a rope will put a heavy load on the drum, although the load of winding a rope will not be so heavy. More specifically, the rope would not move smoothly particularly sideways in a direction parallel to the axis of the drum in the rope storage area and the force F2 required for the rope to slidingly move in that direction eventually exceeds the force (winding force) F1 applied to the rope to move it longitudinally. As a result, the rotation of the rotary drum comes to be often disrupted, emitting shaky rattling noises along with large banging noises that would be generated when the portion of the rope being wound up by the drum abruptly slides sideways in a direction parallel to the axis of the drum. Particularly in the case where the operation of the apparatus for driving a rod antenna element for expansion/contraction is controlled electronically as a function of the intensity of the electric current running through the motor circuit, any excess current running through the motor circuit is detected by the current sensor of the control system to disconnect the power supply of the motor even when the antenna element is on the way of being contracted if the increase in the load of winding the rope is too large.